This invention relates to fluorosilicone rubber compositions curing into products having improved swell resistance upon immersion in fuel oil and lubricating oil as well as satisfactory compression recovery and mechanical strength and suitable for use as seals against such oils as fuel oil and lubricating oil.
Because of heat resistance, freeze resistance, oil resistance, fuel oil resistance, and compression recovery, fluorosilicone rubber is widely used as parts for automobiles, aircraft and other transporting vehicles and parts for petroleum-related equipment.
When rubber parts molded into gaskets and O-rings are kept immersed in fuel oil or lubricating oil during their service as seals, they will swell to increase their volume. If this volume increase is substantial, the rubber part lies out of the sealing area and in an extreme case, is disengaged from the sealing area, failing to provide a sealing function. Also substantial swelling can cause further deformation of the rubber part which has been caulked for seal, and a loss of strength, leading to failure of the part.
It is then necessary to select, as the sealing material against fuel oil and lubricating oil, rubber of the type experiencing minimal swell with the oil to be sealed. For the sealing against lubricating oil, silicone rubber, acrylic rubber, chloroprene rubber, nitrile rubber, hydrin rubber and fluororubber are typically used. For the sealing against fuel oil, nitrile rubber, hydrin rubber, fluororubber and fluorosilicone rubber are typically used.
Of these sealing materials, fluorosilicone rubber is useful since it has improved oil resistance and fuel oil resistance as well as excellent heat resistance, freeze resistance and compression recovery. However, the fluorosilicone rubber has the problem that upon immersion in fuel oil, it swells to a somewhat greater extent than fluororubber used in the same application.
One common practice for reducing the swell of fluorosilicone rubber is to add a large amount of filler. More particularly, when reinforcing silica such as fumed silica or precipitated silica is added in a large amount, rubber hardness increases beyond the practical upper limit and the resulting rubber composition becomes less workable. When non-reinforcing silica such as quartz flour or diatomaceous earth is added in a large amount, the fluorosilicone rubber loses satisfactory mechanical strength such as tensile strength or tear strength.
Increasing a crosslink density is also effective for reducing the swell. With this approach, however, the fluorosilicone rubber increases its hardness beyond the practical upper limit and becomes brittle, losing mechanical strength such as tensile strength or tear strength.
U.S. Pat. No. 5,483,000 and JP-A 6-116498 disclose a further method of blending fluorosilicone rubber with fluororubber for reducing the swell. However, fluorosilicone rubber and fluororubber are rather less miscible with each other. Even when blended, cured parts of their blend do not have sufficient mechanical strength such as tensile strength or tear strength and will delaminate during service. In addition, although fluorosilicone rubber has excellent freeze resistance as compared fluororubber, their blend has as poor a freeze resistance as fluororubber.
None of the above-described methods are satisfactory in improving the swell resistance of fluorosilicone rubber upon immersion in fuel oil.
An object of the invention is to provide a fluorosilicone rubber composition which cures into a product having improved swell resistance upon immersion in fuel oil as well as satisfactory compression recovery and mechanical strength and which is suitable for use as seals against such oils as fuel oil and lubricating oil.
The invention pertains to a fluorosilicone rubber composition comprising an organopolysiloxane of the average compositional formula (1) to be defined below having a viscosity of at least 10,000 centistokes at 25xc2x0 C., a microparticulate silica filler, and a curing agent. To the composition is added a specific amount of a linear organopolysiloxane oil of the general formula (2) to be defined below having trifluoropropylmethylsiloxy groups in its backbone and no crosslinked points in its molecule. The resulting composition cures into fluorosilicone rubber experiencing minimized swell upon immersion in fuel oil and having satisfactory mechanical strength. The invention is predicated on this finding.
Accordingly, the invention provides a fluorosilicone rubber composition comprising
(1) 100 parts by weight of an organopolysiloxane represented by the following average compositional formula (1):
R1aR2bR3cSiO(4xe2x88x92axe2x88x92bxe2x88x92c)/2xe2x80x83xe2x80x83(1)
wherein R1 is trifluoropropyl, R2 is a substituted or unsubstituted monovalent aliphatic unsaturated hydrocarbon group of 2 to 8 carbon atoms, R3 is an unsubstituted, aliphatic unsaturation-free, monovalent hydrocarbon group of 1 to 8 carbon atoms, a is a number of 0.98 to 1.01, b is a number of 0.0001 to 0.01, c is a number of 0.98 to 1.01, and a+b+c is 1.98 to 2.02, and having a viscosity of at least 10,000 centistokes at 25xc2x0 C.,
(2) 5 to 100 parts by weight of a microparticulate silica filler,
(3) 0.5 to 20 parts by weight of an organopolysiloxane represented by the following general formula (2): 
xe2x80x83wherein R4 to R6 are independently selected from substituted or unsubstituted, aliphatic unsaturation-free, monovalent hydrocarbon groups of 1 to 8 carbon atoms, p is a number of 0 to 50, q is a number of 4 to 100, p+q is from 4 to 100, and q/(p+q) is at least 0.7, and
(4) an effective amount of a curing agent.
A first component of the fluorosilicone rubber composition according to the invention is an organopolysiloxane represented by the following average compositional formula (1).
R1aR2bR3cSiO(4xe2x88x92axe2x88x92bxe2x88x92c)/2xe2x80x83xe2x80x83(1)
Herein R1 is trifluoropropyl. R2 stands for substituted or unsubstituted monovalent aliphatic unsaturated hydrocarbon groups of 2 to 8 carbon atoms, preferably 2 to 4 carbon atoms, for example, alkenyl groups such as vinyl and allyl, with vinyl being preferred. R3 stands for unsubstituted, aliphatic unsaturation-free, monovalent hydrocarbon groups of 1 to 8 carbon atoms, preferably 1 to 6 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl and butyl, aryl groups such as phenyl and tolyl and aralkyl groups such as benzyl. Letter a is a number of 0.98 to 1.01, b is a number of 0.0001 to 0.01, c is a number of 0.98 to 1.01, and a+b+c is 1.98 to 2.02.
The organopolysiloxane of formula (1) should have a viscosity of at least 10,000 centistokes (cs) at 25xc2x0 C., preferably at least 50,000 cs at 25xc2x0 C., and more preferably at least 100,000 cs at 25xc2x0 C., so that the silicone rubber obtained by curing the composition may maintain strength. The upper limit of viscosity is not critical, and the organopolysiloxane may be even gum-like.
The organopolysiloxane of the general formula (1) can be prepared, for example, by effecting ring-opening polymerization of tri(trifluoropropyl)trimethylcyclo-trisiloxane using a silisan oligomer shown below as an initiator, as disclosed in U.S. Pat. No. 5,059,668. 
A second component is a microparticulate silica filler. For a practically acceptable mechanical strength, the silica should preferably have a specific surface area of at least 50 m2/g, and more preferably 100 to 400 m2/g as measured by the BET method. Exemplary silica fillers are fumed silica, fired silica and precipitated silica, alone or in admixture of two or more. These silica fillers may be surface treated with surface treating agents such as chain organopolysiloxanes, cyclic organopolysiloxanes, organochlorosilanes, and hexamethyldisilazane.
An appropriate amount of the microparticulate silica filler blended is 5 to 100 parts, preferably 10 to 50 parts by weight per 100 parts by weight of the first component, organopolysiloxane. Outside the range, the composition becomes less workable and cures into a product having unsatisfactory mechanical strength such as tensile strength or tear strength.
A third component is a linear organopolysiloxane oil of the general formula (2) shown below having trifluoropropylmethylsiloxy groups in its backbone and no crosslinked points in its molecule. 
Herein R4 to R6, which may be the same or different, stand for substituted or unsubstituted, aliphatic unsaturation-free, monovalent hydrocarbon groups of 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl and butyl, aryl groups such as phenyl and tolyl, aralkyl groups such as benzyl, and substituted ones of these groups in which some or all of the hydrogen atoms attached to carbon atoms are replaced by halogen atoms or cyano groups, such as chloromethyl, chloropropyl, 3,3,3-trifluoropropyl, and 2-cyanoethyl. Of these, methyl and phenyl are preferred, with methyl being most preferred. Letter p is a number of 0 to 50, preferably 0 to 8, q is a number of 4 to 100, preferably 8 to 80, p+q is from 4 to 100, preferably 8 to 80, and q/(p+q) is a positive number of at least 0.7, preferably at least 0.9. With too small values of q/(p+q), no satisfactory fuel oil resistance is available.
The linear organopolysiloxane oil of formula (2) generally has a viscosity of about 50 to 10,000 cs at 25xc2x0 C.
As the linear organopolysiloxane oil of formula (2), those represented by the following formulae are preferably used alone or in admixture of two or more. Note that Ph is phenyl. 
An appropriate amount of the third component blended is 0.5 to 20 parts, preferably 1 to 10 parts by weight per 100 parts by weight of the first component, organopolysiloxane. Too small amounts of the third component are ineffective for reducing the swell. Silicone rubber compositions with too large amounts of the third component become sticky and less workable, and cure into products having unsatisfactory mechanical strength such as tensile strength or tear strength.
In addition to the first to third components described above, the composition of the invention may contain other optional components. Such optional components include dispersing aids, for example, silanol-terminated siloxanes having a degree of polymerization of up to 100 such as dimethylsiloxane diol and methyltrifluoropropylsiloxane diol, silanol-containing silanes such as diphenylsilane diol and dimethylsilane diol, and alkoxy-containing silanes such as vinyltrialkoxysilanes and methyltrialkoxysilanes; inorganic fillers (other than the microparticulate silica filler as the second component) such as diatomaceous earth, quartz flour, fused quartz powder, clay, alumina, and talc; heat resistance/oil resistance modifiers such as red iron oxide, zinc oxide, titanium oxide, cerium oxide, zinc carbonate, magnesium carbonate, and magnesium oxide; coloring pigments such as carbon black and ultramarine; parting agents; and many other additives commonly added to conventional fluorosilicone rubber compositions. Depending on a particular application, appropriate additives are selected and added in conventional amounts.
The fluorosilicone rubber composition of the invention can be cured by adding a curing agent to the above-described components and effecting vulcanization and cure in a conventional manner. For vulcanization and cure, any of well-known curing agents, preferably organic peroxides may be used. Examples of the curing agent include benzoyl peroxide, tert-butyl perbenzoate, o-methylbenzoyl peroxide, p-methylbenzoyl peroxide, di-tert-butyl peroxide, dicumyl peroxide, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne, alone or in admixture of two or more.
The curing agent is used in an effective amount to induce cure. Typically, about 0.1 to 5 parts by weight of an organic peroxide is used per 100 parts by weight of the silicone rubber composition.
Addition reaction cure is also acceptable, which uses as the curing agent a platinum group catalyst in combination with an organohydrogenpolysiloxane having at least two hydrogen atoms attached to silicon atoms. The platinum group catalyst is preferably used in such amounts as to give about 1 to 2,000 ppm of platinum metal based on the first component. The organohydrogenpolysiloxane is preferably used in such amounts that about 0.5 to 5 SiH groups in the organohydrogenpolysiloxane are available per aliphatic unsaturated hydrocarbon group in the organopolysiloxane as the first component.
The fluorosilicone rubber composition is obtainable by uniformly mixing the above-described components in a conventional manner. The recommended procedure is by premixing the first and second components to form a base compound, optionally heat treating the base compound, and adding the third and fourth components (organopolysiloxane oil and curing agent) thereto.
It is not critical how to mold the fluorosilicone rubber composition. It may be molded into rubber parts of any desired shape such as O-rings, diaphragms and gaskets by any of conventional rubber molding methods such as compression molding, transfer molding, injection molding, extrusion molding, and calender molding. Curing conditions are properly determined in accordance with the curing method employed and the composition. For example, pressure molding is effected at about 150 to 190xc2x0 C. for about 3 to 30 hours. Secondary vulcanization may be effected if necessary.
The cured product of the fluorosilicone rubber composition according to the invention should preferably have a fuel oil resistance represented by a volume change of up to 20% after the fuel oil resistance test of JIS K-6258 prescribing 70 hours of immersion in Fuel C at 23xc2x0 C.
There has been described a fluorosilicone rubber composition which cures into a fluorosilicone rubber having minimized swell upon immersion in fuel oil as well as satisfactory compression recovery and mechanical strength. The rubber finds use as seals like O-rings, diaphragms and gaskets against such oils as fuel oil and lubricating oil.